OP0015 CD38HI CYTOTOXIC CD8 T CELLS ARE EXPANDED FOLLOWING CHECKPOINT BLOCKADE AND CHARACTERIZE THE T CELL INFILTRATE IN CHECKPOINT INHIBITOR-ASSOCIATED ARTHRITIS
Authors: A. Singaraju, K. E. Marks, R. Wang, L. Shakib, G. Dunlap, A. Cunningham-Bussel, S. R. Greisen, L. Chen, A. Tirpack, M. Fein, I. Adejoorin, D. Todd, L. Macfarlane, S. Goodman, E. Dicarlo, E. Massarotti, J. Sparks, A. H. Jonsson, M. Brenner, M. Postow, K. K. Chan, A. Bass, D. Rao, L. Donlin
Journal: Annals of the Rheumatic Diseases (2023)

<h3>Background</h3> Immune checkpoint inhibitor (ICI) therapies used to treat cancer, such as anti-PD1 antibodies, can induce autoimmune conditions. The T cells responsible for mediating such iatrogenic autoimmunity, their activation profiles and characteristics remain unclear. <h3>Objectives</h3> To investigate the transcriptomic and clonotypic features, activation, and role of infiltrating T cells in patient joints affected by ICI-induced inflammatory arthritis (ICI-arthritis). <h3>Methods</h3> Paired TCR and scRNA-seq was performed on synovial fluid (SF) CD8 T cells from ICI-arthritis (ICI-A) to determine their transcriptomic and clonotypic features. Detailed immunophenotyping was performed on SF mononuclear cells using mass cytometry and flow cytometry to identify significantly altered populations in ICI-A compared to seropositive rheumatoid arthritis (RA) and psoriatic arthritis (PsA). Further, bulk RNA-seq was performed on altered SF CD8 T cell subsets from the three arthritides to investigate differences in activation profiles and immune pathways enriched. <i>In vitro</i> cytotoxicity assays were performed by measuring annexin staining in fibroblasts co-cultured with various CD8 subsets from synovial fluid and blood. Mass cytometry was performed on PBMCs from melanoma patients to assess changes before and after checkpoint blockade and bulk TCR-seq of ICI-bound (IgG4+/-) non-naive CD8 T cells was used to analyze TCRs enriched in blood and synovial fluid of ICI-arthritis patients. <h3>Results</h3> Single cell transcriptomic and antigen receptor repertoire analyses highlighted clonal expansion of an activated effector CD8 T cell population in the joints and blood of ICI-arthritis patients (n = 6). These CD8 T cells, identified as CD38<sup>hi</sup>CD127<sup>-</sup>, were uniquely enriched in ICI-arthritis joints compared to RA and PsA and displayed an elevated interferon signature. <i>In vitro</i>, type I interferon induced CD8 T cells to acquire the ICI-associated CD38<sup>hi</sup> phenotype and boosted cytotoxic function in a novel cytotoxicity assay. In a cohort of patients with advanced melanoma, anti-PD-1 and anti-CTLA-4 therapy dramatically expanded circulating CD38<sup>hi</sup>CD127<sup>-</sup> T cells more so than any other T cell population. CD38<sup>hi</sup>CD127<sup>-</sup> T cells were Ki67+ and were frequently bound by the therapeutic anti-PD-1 drug. In ICI-arthritis patients, anti-PD-1 drug-bound CD8 T cells in circulation showed marked clonal overlap with drug-bound CD8 T cells from synovial fluid, but almost no overlap was observed between drug-bound cells in the synovial fluid and non-drug-bound cells in blood (<b>Figure 1</b>). <h3>Conclusion</h3> These results suggest that ICI therapy directly targets CD8 T cells in patients with ICI-arthritis to induce a novel autoimmune pathology that is distinct from the prototypical spontaneous autoimmune arthritides. <h3>Acknowledgements</h3> Funding sources - Rheumatology Research Foundation, Burroughs Wellcome Fund Career Award in Medical Sciences, NIAMS K08 AR072791, P30 AR070253, NIAMS R01, NIAID R01 AI148435. <h3>Disclosure of Interests</h3> None Declared.